In order to improve the mechanical strength of an elongated member such as an electric cable and a fluid feeding pipe, there has been conventionally used a steel wire in the form of armoring member or tensioning member. However, due to the large specific gravity of the steel wire and the resultant heavy weight of the elongated member in which the wire is incorporated, it has been troublesome to carry and lay or install such elongated member. Of late, it has been proposed that the steel wire be replaced by a fiber reinforced plastic wire for reinforcing purposes such as an armoring member, a tensioning member and the like. The fiber reinforced plastic wire is very successful because it has a relatively smaller specific gravity and a relatively larger tensile strength, compared with the steel wire. The greatest disadvantage of the fiber reinforced plastic wire has been that it cannot form a joint having as high tensile strength as welded joint of steel wire. Since the tensile strength of a reinforcing member as a whole depends on that of its joint, the fiber reinforced plastic wire, even though it has a high tensile strength, cannot be used practically for reinforcement if its joint has a lower tensile strength. Conventionally, epoxy resin or cyanide instant adhesive has been used to connect a fiber reinforced plastic wire to another fiber reinforced plastic wire or to a metal wire. In such case the butted faces of the wires are beveled so that they are engaged each other in a relatively larger area and they are covered with and bonded to a sleeve of suitable material by adhesive for improved tensile strength. As a result, the joint of the wires would be undesirably longer. Another disadvantage of such joint is that the cut surfaces must be accurately finished so that they are tightly engaged one another for increased adhesion strength. Further disadvantage is that additional external force must not be applied to the wires to be connected until the adhesive is completely cured, which makes the operation time-consuming. Furthermore, the joint formed by the conventional method in which the two ends are butted together and a FRP sleeve is put and bonded over them, has a large coefficient elasticity and therefore, when used for armoring, must be wound on a cable with a large pitch, thus impairing the flexibility of the cable.